Method of liquefying a hydrocarbon-rich fraction

ABSTRACT

For liquefying a hydrocarbon-rich fraction, in particular natural gas, the hydrocarbon-rich fraction which is to be liquefied is merely cooled and liquefied against a coolant mixture, and the coolant mixture, before its cooling, is separated into a gas phase and a liquid phase, and wherein the gas ( 5 ) and the liquid phases ( 6 ) are cooled (E) separately and not until after cooling (E) has been performed are they expanded (a, b) and recombined ( 8 ).

The invention relates to a method of liquefying a hydrocarbon-rich fraction, in particular natural gas, wherein the hydrocarbon-rich fraction which is to be liquefied is merely cooled and liquefied against a coolant mixture, and the coolant mixture, before its cooling, is separated into a gas phase and a liquid phase.

Methods of the type in question for liquefying a hydrocarbon-rich fraction are used, in particular, in the implementation of natural gas liquefaction processes having a liquefaction power of 200 kW to greater than 20 MW.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are flowsheets, as explained below.

In FIG. 1 a method of the type in question which is comparatively of little complexity in engineering for liquefying a hydrocarbon-rich fraction is shown.

The hydrocarbon-rich fraction which is to be liquefied is fed via line 1′ to a heat exchanger E′ and in this is cooled countercurrently against a coolant mixture which is conducted via line 8′ through the heat exchanger E′, and liquefied. If necessary, a fraction of heavy hydrocarbons is separated off from the hydrocarbon-rich fraction which is to be liquefied in order to adjust the heating value of the liquefied hydrocarbon-rich fraction, and/or to prevent components from freezing out on further lowering of temperature. The liquefied hydrocarbon-rich fraction is subsequently is withdrawn via line 2′ for further use.

The coolant mixture which is completely vaporized during the cooling and liquefaction of the hydrocarbon-rich fraction is fed via line 3′ to a compressor unit V′ which is designed as a single stage or multistage compressor unit and is compressed to the desired cycle pressure which is customarily in a range between 10 and 80 bar. Subsequently, the compressed coolant mixture is fed via line 4′ to a separator D′ and is separated into a gas phase and a liquid phase.

The gas phase is taken off via the line 5′ from the top of the separator D′ and fed to the heat exchanger E′. The liquid phase which is taken off from the bottom of the separator D′ via line 6′ is likewise fed upstream of the heat exchanger E′ by means of the pump P′ and mixed directly in the inlet to the heat exchanger E′ with the gas phase which is fed via line 5′.

The gas and liquid phases which are mixed with one another are passed via line 7′ through the heat exchanger E′ and cooled. In the expansion valve a′ a cold-producing expansion is then performed, before the expanded coolant mixture is again passed through the heat exchanger E′ via the abovementioned line 8′.

This mixing of gas and liquid phases immediately before entry into the heat exchanger E′ is thus useful since it enables the use of a structurally simple and thereby inexpensive heat exchanger. However, it is disadvantageous because the mixing of the gas and liquid phases is comparatively complex, since at least one pump P′ must be provided. Such pumps, however, are relatively high in costs and require maintenance.

It is an object of the present invention to specify a method of the type in question for liquefying a hydrocarbon-rich fraction, in particular natural gas, which avoids the abovementioned problems, and in particular, enables a cheaper and less maintenance-intensive liquefaction process to be implemented.

A method of liquefying a hydrocarbon-rich fraction, in particular natural gas, of the type in question is proposed for achieving this object, which method is characterized in that the gas and the liquid phases are cooled separately and not until after cooling has been performed are they expanded and recombined.

According to the invention the gas and also the liquid phases are then separately cooled to the desired temperature and not combined until subsequently, after expansion to the desired pressure. In this procedure according to the invention the use of a liquid pump can be omitted. The disadvantages associated with such a pump are therefore avoided. Instead of the pump, however, a further expansion valve is required.

The omission of the relatively fault-susceptible pump leads to higher operational reliability and also to longer running time of the liquefaction process. In addition, the operating personnel are relieved, since there is no monitoring of the pumps which were hitherto required. In addition, the method according to the invention leads to a saving of operating costs, in particular maintenance, power and personnel costs.

Further advantageous embodiments of the method according to the invention of liquefying a hydrocarbon-rich fraction, in particular natural gas, are characterized in that

-   -   the gas and liquid phases are recombined before or after warming         thereof against the hydrocarbon-rich fraction which is to be         liquefied,     -   a fraction which is rich in relatively heavy hydrocarbons is         separated off from the hydrocarbon-rich fraction which is to be         cooled,     -   the coolant mixture comprises the components nitrogen, methane,         ethane, ethylene, propane, butane and/or pentane,     -   at least some of the hydrocarbon-rich fraction which is to be         cooled, the fraction which is rich in relatively heavy         hydrocarbons and has been separated off and/or the liquefied         hydrocarbon-rich fraction is added to the coolant mixture,         wherein these fractions preferably replace the component         methane,     -   if the liquefied hydrocarbon-rich fraction is stored         (temporarily) in a storage vessel, the return gas which arises         in the storage vessel is at least in part added to the coolant         mixture, wherein this return gas preferably replaces the         component nitrogen and     -   if the coolant mixture is compressed in at least two stages, the         liquid fraction(s) arising at the or an intermediate pressure         after the cooling of the compressed coolant mixture is or are         pumped to the final pressure of compression and fed to the         separation of the coolant mixture.

The method according to the invention of liquefying a hydrocarbon-rich fraction, in particular natural gas, and also other embodiments of the same may be described in more detail hereinafter with reference to the exemplary embodiment shown in FIG. 2.

As already described with reference to FIG. 1, the hydrocarbon-rich fraction which is to be liquefied is fed via line 1 to a heat exchanger E and therein is cooled and liquefied against a mixed coolant stream which is to be heated and vaporized. Via line 2, the liquefied hydrocarbon-rich fraction is subsequently taken off from the heat exchanger E.

Via line 3, the vaporized coolant mixture which is withdrawn from the heat exchanger E is fed to a compressor unit V which is designed as a single stage or multistage compressor unit and is compressed to the desired final pressure. In correspondence to the selected design of the compressor unit V, liquid can be formed at one or more intermediate stages after the cooling of the compressed coolant mixture, which liquid is pumped to the final pressure of the compressor unit V and is fed to the separator D. The compressed coolant mixture is fed via line 4 to a separator D and is separated into a gas phase and a liquid phase. The gas fraction is taken off via line 5 from the top of the separator D and fed through the heat exchanger E for the purpose of cooling. At the cold end of the heat exchanger E there then proceeds a cold-producing expansion a of this fraction.

The liquid fraction which is taken off via line 6 from the bottom of the separator D is likewise cooled in the heat exchanger E and subsequently thereto is cold-producingly expanded in the valve b. According to the invention, the two fractions are then not combined until after the expansion has been performed in expansion valves a and b and are then jointly fed via line 8 through the heat exchanger E countercurrently to the hydrocarbon-rich fraction which is to be cooled and liquefied.

Alternatively to the procedure shown in FIG. 2, the fractions which are passed through the heat exchanger E via lines 5 and 6 can also only be combined after their repeated passage through the heat exchanger E.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 10 2008 019 392.5, filed Apr. 17, 2008 are incorporated by reference herein.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A method comprising liquefying a hydrocarbon-rich fraction, in particular natural gas, wherein the hydrocarbon-rich fraction which is to be liquefied is cooled and liquefied against a coolant mixture, and the coolant mixture, before cooling, is separated in a separator (D) into a gas phase (5) and a liquid phase (6), the improvement wherein the gas phase (5) and the liquid phase (6) are cooled (E) separately and not until after cooling (E) has been performed are they expanded (a, b) and recombined.
 2. A method according to claim 1, wherein the expanded gas (5) and liquid (6) phases are recombined before or after warming (E) thereof against the hydrocarbon-rich fraction (1) which is to be liquefied.
 3. A method according to claim 1, comprising a preceding step of separating a fraction rich in heavy hydrocarbons from the hydrocarbon-rich fraction which is to be cooled.
 4. A method according to claim 1, wherein the coolant mixture comprises nitrogen, methane, ethane, ethylene, propane, butane and/or pentane.
 5. A method according to claim 3, wherein at least some of the hydrocarbon-rich fraction (1) which is to be cooled, the fraction rich in relatively heavy hydrocarbons and has been separated off, and/or the resultant liquefied hydrocarbon-rich fraction (2) is added to the coolant mixture.
 6. A method according to claim 1, wherein the resultant liquefied hydrocarbon-rich fraction is stored temporarily in a storage vessel, characterized in that recycle gas in the storage vessel is at least in part added to the coolant mixture.
 7. A method according to claim 1, wherein the coolant mixture is compressed to a final pressure in at least two stages, wherein liquid fraction(s) formed at an intermediate pressure after the cooling of the compressed coolant mixture is or are pumped to the final pressure of compression (V) and fed to the separator (D) for separating the coolant mixture.
 8. A method according to claim 2, wherein the expanded gas and liquid phases are recombined before said warming.
 9. A method according to claim 2, wherein the expanded gas and liquid phases are recombined after said warming.
 10. A method according to claim 5, wherein at least some of the methane is replaced.
 11. A method according to claim 6, wherein at least some of the nitrogen is replaced.
 12. A method according to claim 6, wherein the coolant mixture is compressed to a final pressure in at least two stages, wherein liquid fraction(s) formed at an intermediate pressure after the cooling of the compressed coolant mixture is or are pumped to the final pressure of compression (V) and fed to the separator (D) for separating the coolant mixture. 